rust: alloc: add Vec::drain_all

This is like the stdlib method drain, except that it's hard-coded to use
the entire vector's range. Rust Binder uses it in the range allocator to
take ownership of everything in a vector in a case where reusing the
vector is desirable.

Implementing `DrainAll` in terms of `slice::IterMut` lets us reuse some
nice optimizations in core for the case where T is a ZST.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Link: https://lore.kernel.org/r/20250502-vec-methods-v5-4-06d20ad9366f@google.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

Author: Darksonn

rust/kernel/alloc/kvec.rs

@@ -586,6 +586,30 @@ where
             unsafe { ptr::drop_in_place(ptr) };
         }
     }
+
+    /// Takes ownership of all items in this vector without consuming the allocation.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut v = kernel::kvec![0, 1, 2, 3]?;
+    ///
+    /// for (i, j) in v.drain_all().enumerate() {
+    ///     assert_eq!(i, j);
+    /// }
+    ///
+    /// assert!(v.capacity() >= 4);
+    /// # Ok::<(), Error>(())
+    /// ```
+    pub fn drain_all(&mut self) -> DrainAll<'_, T> {
+        // SAFETY: This does not underflow the length.
+        let elems = unsafe { self.dec_len(self.len()) };
+        // INVARIANT: The first `len` elements of the spare capacity are valid values, and as we
+        // just set the length to zero, we may transfer ownership to the `DrainAll` object.
+        DrainAll {
+            elements: elems.iter_mut(),
+        }
+    }
 }
 
 impl<T: Clone, A: Allocator> Vec<T, A> {
@@ -1073,3 +1097,38 @@ where
         }
     }
 }
+
+/// An iterator that owns all items in a vector, but does not own its allocation.
+///
+/// # Invariants
+///
+/// Every `&mut T` returned by the iterator references a `T` that the iterator may take ownership
+/// of.
+pub struct DrainAll<'vec, T> {
+    elements: slice::IterMut<'vec, T>,
+}
+
+impl<'vec, T> Iterator for DrainAll<'vec, T> {
+    type Item = T;
+
+    fn next(&mut self) -> Option<T> {
+        let elem: *mut T = self.elements.next()?;
+        // SAFETY: By the type invariants, we may take ownership of this value.
+        Some(unsafe { elem.read() })
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.elements.size_hint()
+    }
+}
+
+impl<'vec, T> Drop for DrainAll<'vec, T> {
+    fn drop(&mut self) {
+        if core::mem::needs_drop::<T>() {
+            let iter = core::mem::take(&mut self.elements);
+            let ptr: *mut [T] = iter.into_slice();
+            // SAFETY: By the type invariants, we own these values so we may destroy them.
+            unsafe { ptr::drop_in_place(ptr) };
+        }
+    }
+}